Thermal decomposition behavior and de-intercalation mechanism of acetamide intercalated into kaolinite by thermoanalytical techniques

Thermal decomposition behavior and de-intercalation mechanism of acetamide intercalated into kaolinite by thermoanalytical techniques

Applied Clay Science 114 (2015) 309–314 Contents lists available at ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/locate/cla...

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Applied Clay Science 114 (2015) 309–314

Contents lists available at ScienceDirect

Applied Clay Science journal homepage: www.elsevier.com/locate/clay

Research paper

Thermal decomposition behavior and de-intercalation mechanism of acetamide intercalated into kaolinite by thermoanalytical techniques Shenghui Zhang a,b, Xuemei Ou a,⁎, Yinghuai Qiang a, Jinan Niu a, Sridhar Komarneni b,⁎⁎ a b

School of Materials Science and Engineering, China University of Mining and Technology, Xuzhou, 221116, PR China Materials Research Institute, Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802, USA

a r t i c l e

i n f o

Article history: Received 5 March 2015 Received in revised form 2 June 2015 Accepted 4 June 2015 Available online xxxx Keywords: Kaolinite Acetamide Intercalation compound De-intercalation Thermokinetics

a b s t r a c t De-intercalation is the inverse of the intercalation process, which can be easily and rapidly determined by thermal analysis. Research on the de-intercalation process is beneficial to exploring the intercalation mechanism that is still unclear in the case of kaolinite intercalation. The objectives of this study were (a) to investigate the thermal decomposition behavior of the intercalation compound of kaolinite and (b) to determine the kinetics of deintercalation process by thermal analysis to study the mechanism of de-intercalation reaction in kaolinite. Here, the kaolinite–acetamide intercalation compound that was prepared by the direct intercalation method was investigated. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy results showed that acetamide molecules were inserted into the interlayer space of kaolinite and apparently formed new hydrogen bonds with the inner surface hydroxyl groups of kaolinite. The basal spacing of kaolinite increased from 0.721 to 1.102 nm upon intercalation with acetamide. The thermogravimetric (TG) and differential scanning calorimetry (DSC) curves indicated that the decomposition process of the intercalation compound could be divided into two steps. The first step was attributed to the de-intercalation of the intercalated molecules at a temperature of about 181 °C, and the second step corresponded to the dehydroxylation of kaolinite at a temperature of about 502 °C. The completed kinetic triplet of the de-intercalation reaction was obtained through thermal analysis kinetics methods. The apparent activation energy Ea of the de-intercalation process was calculated to be about 73.6 kJ mol−1 by an iterative procedure. The pre-exponential factor A was estimated to be in the range of 1.06 × 1010 s−1 to 7.92 × 1010 s−1 by the Dollimore method. The optimized mechanism function of the deintercalation process of acetamide was determined to be an nth-order chemical reaction through the Malek method. The mechanism function is G(α) = [1 − (1 − α)1 − n] / (1 − n), f(α) = (1 − α)n and experiments showed that the value of n increased with increased heating rate. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Recently, there has been a growing interest in preparing organic– inorganic nanocomposites by intercalation (Solin, 1997; Mittal, 2009). Traditional materials, such as naturally occurring clay minerals, have been widely used in intercalation research, due to their special layer structure. Kaolinite is a member of the 1:1 type layered silicates consisting of one silica tetrahedral [SiO4] sheet combined with an alumina octahedral [AlO6] sheet. The 1:1 layers are stacked together with little or no interlayer space between the layers, but can accommodate some molecules to form organic–inorganic intercalation compounds that may greatly improve the performance and value of kaolinite. Such compounds have a wide variety of applications as novel functional materials because of its asymmetric layer structure and nonlinear optical characteristics (Takenawa et al., 2001; Wang et al., 2005). In addition, ⁎ Corresponding author. Tel./fax: +86 516 83591871. ⁎⁎ Corresponding author. Tel.: +1 814 865 1542; fax: +1 814 865 2326. E-mail addresses: [email protected] (X. Ou), [email protected] (S. Komarneni).

http://dx.doi.org/10.1016/j.clay.2015.06.002 0169-1317/© 2015 Elsevier B.V. All rights reserved.

they can serve as promising precursors for preparing clay–polymer nanocomposites by replacing the intercalated molecules with polymers (Gardolinski et al., 2000a; Pomogailo, 2000). Until now, various organic species such as formamide (Frost et al., 2000), dimethylsulfoxide (Olejnik et al., 1968), methanol (Komori and Sugahara, 1998), Nmethylformamide (Olejnik et al., 1971a), etc. have been reported to be intercalated into the interlayer space of kaolinite. However, most of the intercalation processes take a very long time, at least above 24 h, to reach equilibrium. Also, most of the molecules are difficult to insert into the interlayer space because of the strong hydrogen bonds with no space or exchangeable cations in the kaolinite structure (Brindley and Brown, 1980). Therefore, it is necessary to investigate and understand the intercalation mechanism of kaolinite in order to synthesize many new inorganic–organic nanocomposites for potential applications. However, the complexity of the nano-scale process of kaolinite intercalation makes it very difficult to monitor and explore the mechanism of intercalation by existing techniques. The reverse process of intercalation however, i.e., de-intercalation can throw new light on the intercalation mechanism.

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On one hand, de-intercalation is the inverse process of the intercalation that is rapid and can be easily investigated by thermal analysis (Kristof et al., 2002; Qin et al., 2007). Some researchers previously investigated the intercalation compounds using thermal analysis methods (Horte et al., 1988; Kristof et al., 1999; Lapides and Yariv, 2009; Cheng et al., 2014) but most of these studies are focused on the thermal stability of the compounds with few or none studying the mechanism. Using thermal analysis techniques and determining the kinetics of the deintercalation reaction will open up new avenues for studying the mechanism of the intercalation process. On the other hand, to achieve intercalation of polymer, the precursor clay–organic nanocomposite and the polymer require a match with respect to their thermal properties (Pomogailo, 2000). During the de-intercalation reaction a large amount of intercalated guest molecules may be set free from the precursor before the polymer could melt and intercalate in the interlayers. Rapid release of interlayer organic molecules can lead to little or no intercalation of polymer. Therefore, reaction control, i.e. a careful temperature control during the de-intercalation reaction, is essential for the production of high-quality polymer composites. Understanding the de-intercalation kinetics will result in a control of the reaction rates and provide a theoretical basis for the preparation of kaolinite–polymer composites. In this work, the structure of the kaolinite–acetamide intercalation compound, its thermal decomposition behavior and de-intercalation reaction were investigated by XRD, FTIR, DSC and TG techniques and kinetics. The objectives of this research were (1) to study the thermal decomposition behavior of the kaolinite–acetamide intercalation compound; (2) to investigate the mechanism of the de-intercalation reaction of acetamide from kaolinite and (3) to develop thermokinetic methods for the de-intercalation process. 2. Materials and methods 2.1. Materials The sample used in this experiment was a pure kaolinite from Commerce Company of China University of Geosciences. Laser particle size analyzer showed its average diameter was 2.56 μm. Methanol and acetamide (Ac) were purchased from Shanghai Sinopharm Company, China with purity of A.R. grade. All chemicals were used directly without further purification. Deionized water was obtained from a Milli-Q system, which was stored at b 5 °C and used within a month of preparation.

3. Results and discussion 3.1. Crystal structures of the prepared intercalation compound The two-dimensional layer structure of kaolinite crystals can accommodate organic molecules to form intercalation compounds. When the kaolinite was inserted with Ac, the distance between the layers will increase, which can be determined by XRD through the d001-value directly. The XRD patterns of the kaolinite and its Ac intercalation compound (Kaol-Ac) are shown in Fig. 1. The typical pattern of the kaolinite exhibits a characteristic reflection with a d001-value of 0.721 nm. When the kaolinite was inserted with Ac molecules, its d001-value increased to 1.102 nm, which is consistent with the literature data (Frost et al., 1999, 2002). Noticeably, there was no evidence for crystalline acetamide in the X-ray diffractogram of Kaol-Ac, indicating that the only crystalline materials found within the matrix were kaolinite and Kaol-Ac. The intercalation rate of the compound was calculated to be 61.2% according to the formula, IR = I(001)(c) / [I(001)(c) + I(001)(k)] (Gardolinski et al., 2000b), where I(001)(c) is the intensity of the 001 reflection of the intercalate compound, and I(001)(k) is the intensity of the 001 reflection of kaolinite.

3.2. FTIR spectroscopy FTIR spectra of the untreated and Ac intercalated kaolinites together with the spectrum of Ac in full spectral regions and several characteristic vibration regions are presented in Fig. 2. There was no residual unintercalated acetamide in the final compound as indicated by the absence of characteristic infrared vibrations of pure acetamide in FTIR spectrum. These results are in agreement with the XRD results given above. The presence of interlayer Ac was detected on the FTIR spectrum (Fig. 2A) by the C=O stretching at 1674 cm−1 and by the modification of the inner surface OH stretching mode (Farmer, 1974) at 3697, 3670 and 3651 cm−1 (Fig. 2B) due to interactions of the C=O groups in acetamide with the surface Al–OH group of the clay. The blue shift to 3701 cm− 1 and decrease of the intensity of the band at 3697 cm−1 are related to the interaction of the inner surface hydroxyl with the C = O group in the acetamide molecule (Brindley and Brown, 1980). Clearly two new bands were observed in the FTIR spectra of the intercalated kaolinites at 3481 cm−1 and 3373 cm−1 (Fig. 2C). These bands are

2.2. Preparation of intercalation compounds Firstly, 4.0 g of kaolinite was mixed with 20.0 g Ac by grinding carefully. The mixture was heated up to 85 °C in oil bath and was then stirred at this temperature for 48 h. After cooling down to room temperature in air, the resulting mixture was washed by methanol. The final product was allowed to dry at 60 °C for 12 h. 2.3. Characterization Powder X-ray diffraction patterns of different materials were obtained with a diffractometer (Rigaku D/max-β B) using Cu Kα radiation (λ = 0.15432 nm) at the scanning rate of 8° min− 1 in the two theta range of 3° to 45° with a step size of 0.02° and a counting time of 1 s. The FTIR spectra were recorded using a spectrometer (Bruker Tensor 27), in the range of 4000 to 400 cm−1 with a resolution of 4 cm−1. Samples (ca. 5 mg) were mixed with 100 mg dehydrated KBr and ground together. One hundred scans were collected using KBr pellets at room temperature. Simultaneous TG-DSC measurements were carried out on a thermal analyzer (Netzsch STA 409 PC) under a flowing nitrogen atmosphere (30 mL/min). Approximately 20 mg of the samples was placed in alumina crucibles and heated from room temperature to 1000 °C at heating rates of 1, 3, 5, 7, 10, 15, 20 or 30 °C/min.

Fig. 1. XRD patterns of kaolinite (Kaol), acetamide (Ac) and kaolinite–acetamide (Kaol-Ac) intercalation compound.

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Fig. 2. FTIR spectra of (A) complete spectra of raw kaolinite (Kaol), pure acetamide (Ac) and kaolinite–acetamide intercalation compound (Kaol-Ac). (B) Kaolinite OH stretching band region. (C) Stretching of NH2. (D) Stretching of C=O (E) in plane vibration of Si–O.

attributed to the symmetric and asymmetric stretching frequencies upon formation of the N–H…O–Si units (Olejnik et al., 1971b). Correspondingly, the C=O symmetric stretching at 1683 cm−1 in the pure solid acetamide was observed in the intercalated kaolinite at 1674 cm−1 (Fig. 2D) while the Si–O in plane vibrations at 1112 cm−1, 1032 cm−1 and 1005 cm−1 were shifted to higher wavenumbers with the shape of the band changed (Fig. 2E). These changes are also evidence for two types of interactions between the Ac and kaolinite surfaces i.e., the Ac molecule is hydrogen bonding to both the siloxane and gibbsite-like kaolinite surfaces simultaneously. 3.3. Thermal analysis The results of the thermal analysis for the raw kaolinite and its intercalation compound are displayed in Fig. 3. The TG-DSC curves of the untreated kaolinite (Fig. 3A) presented only one mass loss step with a mass loss of 11.78% within the temperature range of 476– 560 °C with only one endothermic peak at 496 °C, the latter is attributed to the dehydroxylation of kaolinite. Compared with pure kaolinite, the TG-DSC curves of Kaol-Ac (Fig. 3B) possessed two mass loss steps and two endothermic peaks. The first mass loss step with a mass loss of 6.28% at about 181 °C corresponds to the de-intercalation of the interlayer Ac molecules. The second mass loss step at 502 °C, which is

accompanied by a mass loss of 11.09%, is associated with the dehydroxylation of de-intercalated kaolinite. However, the temperature of the dehydroxylation of de-intercalated kaolinite was lower than that of the untreated kaolinite by approximately 6 °C. This change may due to the striking structural degradation of kaolinite caused by the intercalation process. The second mass loss (11.09%) of the intercalated compound is relative to the total weight of the compound. It can be calculated as 11.80% with respect to the amount of the kaolinite in compound, which is very close to the mass loss (11.78%) resulting from the dehydroxylation of raw kaolinite. Thus, it can be presumed that Ac was completely deintercalated in the first step, and the compound decomposition includes de-intercalation and dehydroxylation as two separate decomposition processes. The amount of Ac in the compound was 6.28% from which an intercalation rate of 61.2% was calculated. Therefore, the molar ratio of Ac and kaolinite in the compound was 1:2.09. The compound de-intercalation reaction was: [Al2(Si2O5)(OH)4] 2.09 (C2H5NO) (s) → 2.09 Al2(Si2O5)(OH)4(s) + C2N5NO(g) The de-intercalation kinetics of the first step was further studied below.

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Fig. 3. Thermal analysis curves (TG-DSC) of (A) kaolinite and (B) kaolinite–acetamide intercalation compound at 10 °C min−1 heating rate.

3.4. Kinetics studies of the de-intercalation 3.4.1. Activation energy Eα In the non-isothermal conditions, decomposition kinetics formula is: dα=dT ¼ ðA=βÞ expð−Ea =RT Þf ðα Þ; which β ¼ dT=dt

ð1Þ

Transform the Eq. (1): dα=f ðα Þ ¼ ð1=βÞA expð−Ea =RT ÞdT

ð2Þ

Integrate the Eq. (2): Z Gðα Þ ¼

a 0

  KAS iterative : ln β=Q 4 ðxÞT 2 ¼ ln ðAR=Ea Gðα ÞÞ−Ea =RT

ð4Þ

Ozawa iterative : ln ðβ=H ðxÞÞ ¼ ln ð0:00484AEa =RGðα ÞÞ−1:0516Ea =RT ð5Þ

Z da=f ðaÞ ¼ 1=β

different heating rates with the same conversion rate. Since the calculation of these two methods does not involve the choice of the mechanism functions, Ea value can be directly calculated corresponding to the reaction at different α. But it would produce a certain deviation in the calculation because of the integral approximation. Therefore the iterative method (Gao et al., 2001) was used to calculate the activation energy accurately. The functions of the iterative method are as follows:

T T0

A expð−Ea=RT ÞdT ¼ ðAEa =βRÞP ðxÞ ð3Þ

where T0 is the origin temperature, T is the completed temperature, α is the conversion rate, x = Ea/RT. Substitute P(x) = 0.0048exp (−1.0516x) and P(x) = exp (−x)/x2 into Eq. (3), the classic Ozawa method and the KAS method formula (Hu and Shi, 2001) of the corresponding equation were obtained. The activation energy Ea can be calculated from the slopes of the lines, which were obtained by a linear regression lnβ and ln(β/T2) to 1/T at

The definition of Q4(x) and H(x) is:     Q 4 ðxÞ ¼ x4 þ 18x3 þ 88x2 þ 96x = x4 þ 20x3 þ 120x2 þ 240x þ 120

ð6Þ

H ðxÞ ¼ expð−xÞQ 4 ðxÞ=x2 0:0048 expð−1:0516xÞ

ð7Þ

The procedure to calculate Ea can be divided into the following three steps by iteration: (a) Firstly, setting H(x) = 1 or Q4(x) = 1, the value of Ea0 was calculated by least squares method according to the slope of the linear

S. Zhang et al. / Applied Clay Science 114 (2015) 309–314 Table 1 Ea of de-intercalation reactions with different α by KAS, Ozawa and iterative methods. Ea/(kJ · mol−1)

α

KAS 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90

70.43 74.80 78.18 78.64 77.76 75.86 72.93 68.04 63.52

Ozawa 73.48 77.72 81.06 81.59 80.84 79.12 76.42 71.87 67.65

ln(β / H(x)) − 1 / T 70.71 75.07 78.45 78.91 78.04 76.16 73.24 68.38 63.88

ln(β / Q4(x)T2) − 1 / T 70.70 75.06 78.44 78.91 78.04 76.15 73.24 68.37 63.88

relationship lnβ − 1 / T and lnβ/T2 − 1 / T. (b) Ea0 and T at different α and β were substituted into x = Ea / RT, then the x was calculated and substituted into H(x) and Q4(x) according to different β. Then the obtained values of H(x) and Q4(x) were substituted into Eqs. (4) and (5), from which the value of Ea1 was calculated by least squares method according to the slope of linear relationship ln(β/H(α)) − 1 / T and ln(β / Q4(x)T2) − 1 / T. (c) Replaced initial Ea0 with the calculated E a1 , repeat step (b) until E ai − E ai-1 b 0.01 kJ mol − 1 (Gao et al., 2001; Qin et al., 2007), then the obtained last value Eai is the exact value of activation energy of the de-intercalation reaction. The results of Ea which were calculated by Ozawa, KAS and iterative methods are listed in Table 1. As can be seen, the calculated results obtained by the Ozawa method showed large deviation compared with the other two methods. However, results from KAS method were closer to the results from iterative methods i.e., the deviation was from −0.82 to −0.33 kJ mol−1 only. The results of two iterative methods are basically the same (Table 1). Because the two iterative methods are an improvement over those of the Ozawa and KAS methods, the average Ea = 73.6 kJ · mol−1 from the two iteration methods can be thought of as the activation energy of the de-intercalation reaction. 3.4.2. Mechanism functions The function of de-intercalation reaction mechanism was studied by the Malek method (Malek and Smrcka, 1991) which was defined by y(α): yðα Þ ¼ ðT=T 0:5 Þ2 ðdα=dt Þ=ðdα=dt Þ0:5 ¼ f ðα Þ  Gðα Þ= f ð0:5Þ  Gð0:5Þ

ð8Þ

Table 2 Results of experimental data and 3 mechanisms at different heating rates by the least squares method. β/(°C min−1)

No. 31 function

No. 9 function

No. 43 function

1 3 5 7 10 15 20 30

0.4508 0.5514 0.7208 0.9375 0.5387 0.0452 0.6339 0.8682

0.5598 0.4247 0.3193 0.1925 0.3958 0.3127 0.3110 0.4285

0.0866 0.0863 0.1138 0.1187 0.0888 0.0939 0.1352 0.1151

y(α)–α curve, and these curves can be seen as experimental curves. If experimental curves match with the standard curves, then the corresponding curves of f(α) and G(α) can be determined as the most probable mechanism functions. The standard curves could be obtained according to the right side of Eq. (8) which includes 45 mechanism functions of f(α), G(α) (Hu and Shi, 2001). The standard curves were drawn by first omitting curves which did not fit from the 45 mechanism functions. Only No. 9, No. 31, and No. 43 mechanism functions of the standard curves had small deviations when compared with the experimental curves as shown in Fig. 4. In order to determine the probable mechanism functions further, the least squares method was adopted in this paper to calculate the deviation of the experimental curves and the standard curves quantitatively. The results are listed in Table 2. The results reported in Table 2 show that the experimental curves of the de-intercalation reaction of kaolinite-Ac intercalation compound are closest to the curve of the No. 43 mechanism functions. Thus the No. 43 mechanism function is the most probable mechanism function, which is n-order chemical reaction: h i Gðα Þ ¼ 1−ð1−α Þ1−n =ð1−nÞ; f ðα Þ ¼ ð1−α Þn

ð9Þ

There is an n in the No. 43 mechanism function, which may be altered by heating rate. To determine its value at different heating rates, the error sum of squares was calculated to determine which were the smaller values by the least squares method. Different values of n were substituted into No. 43 mechanism function and the results are shown in Fig. 5. The value of n showed a relationship with the heating rate and increased with the increase in heating rate. When n was equal to 0.9 the heating rate was 1 °C min− 1 but it increased rapidly to 1.6

n

Substitute the pre-established data into the right of the Eq. (8), drawing the curve y(α)–α, and these curves can be seen as standard curves. Substitute the experimental data into the left of Eq. (8), drawing

313

Fig. 4. Experimental curves and the standard curves at 10 °C min−1 heating rate.

Fig. 5. Results of n in No. 43 function at different heating rates by the least square method.

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to the de-intercalation of intercalated Ac molecules, whereas the process at 502 °C was associated with the dehydroxylation of the deintercalated kaolinite. The most probable mechanism of the reaction is n-level chemical reaction, and n in the mechanism function, which increased with increasing heating rates. Structural and thermal analyses and kinetics calculation showed that strong chemical bonding existed between intercalated molecules and the inner surface of kaolinite. It can be inferred that intercalation process is a chemical reaction process, which is the driving force of intercalation reactions. These studies are beneficial to understand the mechanism of de-intercalation process of acetamide organic molecule from kaolinite as well as provide a new method and experimental data to infer the intercalation mechanism. Acknowledgments This work was financially supported by the National Natural Science Foundation of PR China (61107080), Visiting Scholarship Fund of China Scholarship Council (20133050) and Fundamental Research Funds for the Central Universities (2013QNB03).

Fig. 6. Experimental and standard curve of y(α) vs. α.

with the heating rate of 5 °C min−1. However, when heating rates were greater than 5 °C min−1, n increased only slightly. The experimental and standard data with No. 43 function (n = 1.9) at a heating rate of 30 °C min−1 are compared in Fig. 6. The results (Fig. 6) show that the experimental data are in good agreement with the standard curve, which suggests that the mechanism functions obtained here are accurate. 3.4.3. Pre-exponential factor A Dollimore et al. (1996) replaced the Arrhenius rate constant k by rate constant formula k = CTm of Harcourt–Esson type (H–E type) and derived the H–E differential formula of constant C and m. The H–E differential formula is:   lg ðdα=dT Þi β= f ðα Þi ¼ lgC þ m lgT m ði ¼ 1; 2; …; jÞ

ð10Þ

The (dα / dT)i, β, f(α)i, Ti and αi should be substituted. Because of dα / dT = (dα / dt)(dt / dT) the value of (dα / dT)i could be obtained from (dα / dt)i divided by the corresponding β. To derive the f(α)i which corresponds to αi, just substitute αi into the No. 43 mechanism function. The above data were substituted into formula (10), then lgC and m could be calculated from the slopes and intercepts of the plot of lg[(dα / dT)iβ / f(α)i] versus lgTi. At last, lnA and A (as shown in Table 3) at the respective heating rates could be obtained by the Arrhenius relationship lnki = lnA − Ea / RTi. From the calculation, the range of pre-exponential factor A is 1.06 × 1010 s−1 to 7.92 × 1010 s−1. In summary, the kinetics calculation showed that the most probable mechanism of the de-intercalation reaction is n-level chemical reaction, and n in the mechanism function which increases with increasing heating rates. The apparent activation energy Ea was about 73.6 kJ mol−1. These imply that the de-intercalation is a chemical reaction process because of the strong chemical bonding that exists between intercalated molecules and the inner surface of kaolinite. 4. Conclusions Acetamide inserted into the interlayers of kaolinite appears to have formed new hydrogen bonds with the inner surface of kaolinite. Two separate decomposition processes were observed with the heating temperatures at 181 °C and 502 °C. The process at 181 °C was attributed Table 3 Results of lnA and A at different heating rates. A and lnA

lnA A × 10−10

β/(°C · min−1) 1

3

5

7

10

15

20

30

23.57 1.73

24.41 4.00

23.08 1.06

24.74 5.57

25.09 7.92

24.67 5.17

24.48 4.26

23.49 1.59

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